Intervertebral implant

ABSTRACT

A hollow cylindrical intervertebral implant with a longitudinal axis ( 3 ), a top surface ( 1 ) and a bottom surface ( 2 ), consisting essentially of a ceramic material displaying a maximum porosity of 30 vol. %, the pores of which are filled with air. The implant according to this invention is characterized in that upon primary fusion during the resorption process it equalizes the distance (corresponding to intervertebral disk height) between two vertebrae while providing adequate fusion and is resorbed by the body after a certain amount of time.

[0001] This invention relates to an intervertebral implant as specifiedin claim 1.

[0002] Intervertebral implants of this type are inserted when, afterremoval of the intervertebral disk between two vertebrae especially inthe lumbar section of the vertebral column, these vertebrae are to befused. One or two such implants are used in each intervertebral space.

[0003] In EP-B 346.269, FUHRMANN ET AL already describe anintervertebral implant whose outside front, end and lateral surfaces arecoated with a layer of hydroxyl apatite or a ceramic HIP material. Thedrawback of this earlier implant lies in the fact that the basic body ofthe implant consists of typical nonceramic and hence nonresorbablematerials.

[0004] In U.S. Pat. No. 5,306,303, LYNCH describes an intervertebralimplant which consists entirely of a porous ceramic material. Thedrawback of that earlier implant concept lies, on the one hand, in itslow pressure resistance attributable to the relatively high porosityand, on the other hand, in the fact that the implant cannot be filledwith bone chips with which to obtain accelerated bone integration.

[0005] Another intervertebral implant has been described in EP 505 634by OKA et al, consisting of a porous ceramic base element with hydrogeldeposited in the pores. This earlier implant as well offers insufficientpressure resistance owing to the hydrogelfilled pores.

[0006] In EP-A-493 698, HARLE describes a bone substitute for fillingfault areas, consisting of two different, porous, ceramic materials thepores of which are evacuated.

[0007] Finally, DE-A 44 23 826 by ASAHI describes an artificial ceramicvertebra the porosity of which is maintained by means of a foamingagent.

[0008] This invention is intended to solve the problem. Its objective isto provide an intervertebral implant that can hold up to the variouspressures to which the vertebral column is exposed, offering asufficiently large contact surface at the end plates so as to preventthem from sinking in. It is also designed to permit fastest possiblefusion of the two vertebrae as well as rapid incorporation of theimplant with due allowance for the height of the intervertebral diskprior to its removal. In a subsequent progression, the implant should befully (or nearly fully) capable of being replaced by the patient's ownbone growth.

[0009] The characteristic features of the independent claim 1 providethe enhancements to the implant referred to above that are necessary to,solve the problem.

[0010] Advantageously, this makes it possible for the implant accordingto this invention, upon primary fusion during the resorption process, toequalize the distance (corresponding to intervertebral disk height)between two vertebrae while providing adequate fusion and to be resorbedby the body after a certain amount of time to a point where it is nolonger detectable.

[0011] Another major advantage of this implant is its transparency toxrays, which avoids artifacts that would interfere with a diagnosis ofthe surrounding bone structure.

[0012] The intervertebral implant may be shaped as a prismatic or as acylindrical element, with a porosity not to exceed 30% by volume. In onepreferred enhanced embodiment of this invention, the porosity of theceramic material is 9 vol. % at the most and preferably not more than 5vol. %. Reduced porosity of the implant provides greater pressureresistance which is a fundamental requirement especially in the lumbarsection of the vertebral column. In this area, particular importance isattributed to the largest possible contact surface between the end plateand the implant. Therefore, the wall thickness of the ring-shapedintervertebral implant should be at least 4 mm and preferably at least 6mm so as to inhibit any penetration, of the implant into the end plates.

[0013] In another preferred embodiment of this invention, the ceramicmaterial has a density value of greater than 2.8 and preferably greaterthan 3.1, which further enhances the pressure resistance of the implant.

[0014] The implant is preferably configured as a hollow, circularcylinder which permits the insertion of the patient's own bone chips orsimilar biocompatible material, thus promoting rapid fusion of theimplant.

[0015] In another preferred embodiment of this invention, the topsurface and/or the bottom surface of the implant is not planar but isprovided with grooves and/or ridges extending perpendicular to the axisof the cylinder. Such three-dimensional structuring of the top andbottom surface would permit primary fastening of the implant immediatelyafter its introduction in the intervertebral space, thus enhancing thepositional stability of the implant and the rotational stability of theadjoining vertebrae. The three-dimensional surface structure ispreferably in the form of “undulations” (raised reinforcing ridges withdistinct radii) in both the longitudinal and horizontal directions.

[0016] Depending on where the implant is applied, the top and/or bottomsurface extend parallel or in wedge-like converging fashion in relationto each other so as to permit adequate following of the curvature(lordosis, kyphosis).

[0017] The implant is preferably provided with a convex top and/orbottom surface which matches the concave shape of the natural end platesof the vertebrae, to achieve better contact between the implant and theend plates.

[0018] The jacket of the intervertebral implant is preferably providedwith one or several perforations primarily for the purpose of engagingan instrument for manipulating the implant. The perforations may belocated both on the anterior side and in the lateral zone of theimplant. The perforations additionally serve to facilitate primary bonegrowth through the implant.

[0019] The positional stability of the implant can be further improvedby providing the jacket of the intervertebral implant with a fine,three-dimensional texture which promotes bonding with the bone at anearly stage. This textural structure is preferably 0.5-1.0 mm deep, withgrooves 0.5 to 1.0 mm wide. The entire surface of the jacket may betextured in that fashion.

[0020] For the implant according to this invention, suitable, ceramicmaterials may be used which have typically and successfully been used inmedicine, except with a porosity as defined by this invention, withpreference given to polycrystalline ceramics with a foreign-phasecontent of less than 3 and preferably less than 2% by weight. Thepressure resistance of the ceramic material should be between 400 and600 MPa and preferably between 450 and 550 Mpa.

[0021] The following will describe the invention and its enhancedimplementations in more detail with the aid of a partly schematicillustration of an embodiment shown by way of example.

[0022] Shown in the sole FIGURE is

[0023] a perspective view of the implant according to this invention.

[0024] The intervertebral implant illustrated in this one FIGUREconsists essentially of a hollow cylinder with an inner space 8, alongitudinal axis 3, a top surface 1 and a bottom surface 2. Theintervertebral implant is essentially produced from a polycrystallineceramic material. The ceramic material has a porosity of 5 vol. %, thepores are filled with air. The width of the pores is less than 100 μmand preferably less than 50 μm. The foreign-phase content of the ceramicmaterial is 1.5% by weight. The pressure resistance of the ceramicmaterial is 500 Mpa.

[0025] The top and bottom surfaces 1, 2 serve to provide bone contactwith the surface plates of two vertebrae and are configured accordingly.The wall thickness of the intervertebral implant is 7 mm, the density ofthe ceramic material is 3.2. The top surface 1 and the bottom surface 2are not planar but are provided with a number of grooves 4 and ridges 5extending in a perpendicular (i.e. radial) direction relative to thelongitudinal axis 3.

[0026] The top surface 1 and the bottom surface 2 extend in wedge-likeconverging fashion in relation to each other and have a slightly convexoutward curvature.

[0027] The anterior side of the jacket 6 of the intervertebral implantis provided with a perforation 7 serving to accept a manipulatinginstrument. The jacket is further provided with a three-dimensionalsurface structure 9 having a depth of 0.75 mm.

[0028] The following will describe in detail the clinical application ofthe intervertebral implant according to this invention.

[0029] The implant illustrated in the FIGURE is filled with bone chips(bone graft or bone substitutes), possibly compressed, grasped with asuitable instrument by insertion in the perforation 7 and introducedinto the appropriately cleared-out intervertebral space with the aid ofa distractor device.

1. Intervertebral implant, configured in prismatic or cylindrical formwith a longitudinal axis (3), its top surface (1) and bottom surface (2)designed to serve as contact surfaces between the bone and the endplates of two vertebrae, the said intervertebral implant consistingessentially of a porous ceramic material whose porosity is 30 vol. % atthe most and whose pores are filled with air, characterized in that thejacket (6) of the intervertebral implant is provided with athree-dimensional surface structure (9).
 2. Implant as in claim 1,characterized in that the said threedimensional surface structure (9)has a depth of 0.5 to 1.0 mm.
 3. Implant as in claim 1 or 2,characterized in that the top surface (1) and/or the bottom surface (2)are/is not planar.
 4. Implant as in one of the claims 1 to 3,characterized in that the top surface (1) and/or the bottom surface (2)are/is provided with grooves (4) and/or ridges (5) extending in adirection perpendicular to the longitudinal axis (3)
 5. Implant as inone of the claims 1 to 4, characterized in that the jacket (6) of theintervertebral implant is provided with one or several perforations (7)6. Implant as in one of the claims 1 to 5, characterized in that theporosity of the ceramic material is 9 vol. % at the most.
 7. Implant asin claim 6, characterized in that the porosity of the ceramic materialis 5 vol. % at the most.
 8. Implant as in one of the claims 1 to 7,characterized in that the wall thickness of the intervertebral implantis at least 4 mm.
 9. Implant as in claim 8, characterized in that thewall thickness of the intervertebral implant is at least 6 mm. 10.Implant as in one of the claims 1 to 9, characterized in that thedensity of the ceramic material is greater than 2.8 g/cm³.
 11. Implantas in claim 10, characterized in that the density of the ceramicmaterial is greater than 3.1 g/cm³.
 12. Implant as in one of the claims1 to 11, characterized in that the intervertebral implant is configuredas a hollow circular cylinder extending along the longitudinal axis (3).13. Implant as in one of the claims 1 to 12, characterized in that thetop surface (1) extends parallel to the bottom surface (2)
 14. Implantas in one of the claims 1 to 12, characterized in that the top surface(1) and the bottom surface (2) extend in wedge-shaped converging fashionin relation to each other.
 15. Implant as in one of the claims 1 to 14,characterized in that the top surface (1) and/or the bottom surface (2)are/is curved in convex fashion toward the outside.
 16. Implant as inone of the claims 1 to 15, characterized in that the ceramic material ispolycrystalline.
 17. Implant as in one of the claims 1 to 16,characterized in that the ceramic material has a foreign-phase contentof less than 3 and preferably less than 2% by weight.
 18. Implant as inone of the claims 1 to 17, characterized in that the pressure resistanceof the ceramic material is in the range from 400 to 600 MPa.
 19. Implantas in claim 18, characterized in that the pressure resistance of theceramic material is in the range from 450 to 550 MPa.
 20. Implant as inone of the claims 1 to 19, characterized in that the width of the poresis less than 100 μm.
 21. Implant as in claim 20, characterized in thatthe width of the pores is less than 50 μm.
 22. Implant as in one of theclaims 1 to 21, characterized in that the ceramic material istransparent to x-rays.